Printing presses for directly printing three-dimensional objects as e.g. bottles or cans are known, which print the surfaces of three-dimensional objects by means of drop-on-demand technology. This type of printing press is also encompassed by embodiments of the present invention.
These known printing presses or systems are in principle all constructed for higher performance so that they print the objects in a transport device, whose transport direction is substantially aligned horizontally in order to move objects from an intake, also called input, to a discharge, also called output. For rotation systems, this means that the rotational axis of such printing presses is vertical to a clocked wheel.
Many such systems operate in such a manner that the print heads with the object bracket, also called holder, are moved along during transport and the print occurs during transport.
However, other printing presses, called timing systems, are also known, in which the print heads are fixed and the objects to be printed are respectively cyclically successively driven under or in front of the print heads and are printed on their surface in a stationary manner, while the object to be printed is e.g. moved by rotation and thus rotates in front of a print head. The objects usually have a cylindrical shape in the region of the surface to be printed. These cylindrical objects are often arranged in the known printing presses with their symmetry axis horizontally, as the print heads were originally developed for flatbed printing. The cylindrical objects rotate under the print head about this horizontal rotation or symmetry axis, in order to have an image applied to them. This is shown in FIG. 1, which shows a printing press 201 with a clocked wheel 202, at whose circumferential edge the three-dimensional objects 203 are held horizontally via a holder so that the three-dimensional objects 203 may rotate under the fixedly arranged print heads 204. These print heads 204 each form a printing station 205 of the printing press 201. The clocked wheel 202 is actuated so that the objects 203 are respectively arranged in the printing stations 205 in a cycle position of the clocked wheel 202. The holders and the print heads 204 are arranged for this in a division corresponding to each other. This also applies in principle for the printing presses according to embodiments of the present invention.
The print direction illustrated in a part of the print heads 204 by lines between the print heads 204 and the objects 203 passes, as is usual with inkjet printers in flatbed printing, vertically from top to bottom, that is, with gravity. This arrangement is often also maintained when printing three-dimensional, in particular (in the printing region) cylindrical objects.
If e.g. cylindrical objects such as bottles or cans shall now be printed with such a printing press by rotation of the objects in front of the print heads, wherein the print image to be printed (in the direction of the rotational axis of the objects) is wider than the printing width of an ink jet head (which is defined by the linear arrangement of the print nozzles in the print head), the objects to be printed and the print head must be moved relative to one another in order to achieve greater printing widths.
However, this takes longer than the printing by rotating once about the axis of symmetry of the object, as a rotation of the object must take place in each relative position of the object and the print head. It is therefore attempted to print several sections of the image simultaneously by arranging several print heads. As the print heads need fixings and housing, a shock-free direct placement of two print heads next to each other is not possible so that the image can be printed smoothly. It is therefore necessary that the images are printed either in two successive printing stations, wherein the second part of the print of the desired print image then takes place in the following printing station. With even wider print images, two spaced-apart image parts can be printed in the first station. The free space between these two partial images of the preceding print head arrangement is then correspondingly printed in the following printing station. In principle, it would also be conceivable to arrange the print heads offset to each other in a printing station to arrange, as is done in a flatbed print. The offset print heads must thereby be aligned to the center of the rotational axis of the object in order to achieve a clean printing image.
The above-described arrangement of several print heads in a printing station is however disadvantageous when printing for example round or cylindrical three-dimensional objects. If such objects are moved along a transport direction in the printing press, they do not allow other parts of the printing press in the transport direction, unless these parts or objects are moved out of the transport plane during the transport. However, print heads cannot be accelerated or moved quickly, as the ink therein is otherwise ejected from the print nozzles and the important printing conditions change within the print head.
If one thus needs more than one print head for a desired print image for printing a surface, these print heads must be arranged either above a transport route or beneath a transport route, as the print head needs to be very close to the surface to be printed. Theoretically, the transport movement of the three-dimensional objects can also be designed so that it does not take place in only one plane, but the object is lifted after the cycle step and is guided to the print heads, or is lowered again before continuing the cycle. This does not only result in a loss of time for the lifting and lowering, but also in a much more complex mechanism and the risk, during movement in two planes, not to find exactly the point so that drop on drop is printed for a pixel of the print image. The results are blurred, qualitatively not high-quality print images.
The arrangement of a second print head below the transport plane is (in the illustration of FIG. 1) indeed theoretically possible. In practice, however, the problem exists that falling dirt particles and sprayed ink residues of the print head arranged above the transport plane immediately contaminate the print head arranged below the transport plane and, after a short time, this would not provide a clean print. Usually, with several colours or a wider print image, correspondingly more stations are provided on the press, in order to be able to arrange the print heads successively on several stations.
While this results in a lower mechanical effort in the printing press, it has the decisive disadvantage that the print image is not homogeneous. If images are printed that are made up of individual drops of for example different colour, the drops flow further apart after the impact on the surface. Thus, the right time between application and pinning or partial curing of the drops (for example by UV irradiation) must be found, so that the desired colour impression due to the surface covered after the flowing apart always remains the same. Such pinning processes are usually performed before the cycle transport step, as the transport step is connected with considerable acceleration, and the still low-viscosity aqueous ink can otherwise form drips by the inertia of the ink mass. At least an attempt is made to reduce the necessary cycle transport steps as far as possible. The respective colour layer could be printed cleanly in two steps and also pinned. Here, however, the fact is overlooked that exposed colour areas are pinned several times. The first of the colours applied in two steps is pinned twice, but the second one only once. The colour is no longer homogeneously the same for the following colour application, but it has two different properties through the different partial cure/the different pinning. These result in surface tension and adhesion of the subsequent colour.
Each additional curing changes the surface tension of a colour, and results, when subsequently printing with other colours, that these become either too hydrophilic or too hydrophobic. The drop of the colour to be applied later thus impinges a surface with different and not foreseen surface tensions and is thus not applied as reproducibly so that a good print image results. As UV colours cure by polymerization, it is important that they cross-link with the underlying colour layer. This is not ensured sufficiently if the underlying layer is already polymerized too much by two curing processes. The colours are then no longer cross-linked, but only lie on top of each other, which is reflected in reduced colour adhesion.